We can all agree that airplanes are cool, and rockets are awesome, but when combined, the result is even better! Besides getting engineers to jump up and down for this revolutionary concept, Reaction Engines Ltd applied it to an actual SABRE engine concept.
SABRE stands for Synergistic Air-Breathing Rocket Engine and one typically does not associate “Air-Breathing” with “Rocket.” which makes this engine a one of a kind to reach new heights (literally). Let’s dig into the geeky technical specs of the engine while going through some quick history of this revolutionary single stage to orbit propulsion system.
SABRE is an evolution of Alan Bond's series of liquid air cycle engine (LACE) and LACE-like designs that started in the early/mid-1980s under the HOTOL project. Upon termination of HOTOL funding, Bond formed Reaction Engines Ltd. SABRE is currently being developed for hypersonic flights and runs on a combined cycle; the precooled jet engine configuration is used in the air-breathing phase of the flight until air becomes scarce and speed critical. From this moment on the engine switches to its close cycle rocket mode to bring the Skylon airplane to orbit (2 engines are mounted on the aerospace plane).
The air-breathing mode (below Mach ~5 and about 25 km altitude which is about 20% of the orbital velocity and altitude, respectively) works almost like a regular jet with one major difference being the apparition of a new component, first discussed in 1955; the air precooler which is placed behind the translating axisymmetric shock inlet cone that slows the air to subsonic speeds inside the air-breathing engine using 2 shock reflections. The precooler is “capable of cooling incoming air (without liquefying it, from around 1000°C) to −150°C (−238°F), to provide liquid oxygen (LOX) for mixing with hydrogen to provide jet thrust during atmospheric flight before switching to tanked LOX when in space." This precooler also allows a considerable reduction of the thermal constraints of the engine which then requires “weaker” and much lighter materials that are a necessity when reaching orbital velocities and altitudes. With compressors working more efficiently with a colder fluid, and the incoming air already highly compressed from the flight speed and shock waves, the fed pressure in the combustion chamber is around 140 atm. When in rocket mode, the inlet cone is closed and liquid oxygen and liquid hydrogen are burned from on-board fuel tanks for the remaining 80% of velocity and climb required to reach orbit.
On a very recent note, feasibility studies conducted by the U.S. Air Force Research Laboratory were successfully passed in 2015.
Although the application of the SABRE engine is destined for orbital use, its cousin (Scimitar) has been designed for the environmental-friendly A2 hypersonic (top speed higher than Mach 5) passenger jet for 300 rushed passengers (about 3 times more than the Concorde) under the LAPCAT (Long-Term Advanced Propulsion Concepts and Technologies) study founded by the European Union.
When dealing with such high speeds, noise becomes a real constraint and flying above inhabited areas is restricted, which is why specific aerial routes are designed. According to Alan Bond, the A2 design could fly subsonically from Brussels International Airport into the North Atlantic, reaching Mach 5 across the North Pole and over the Pacific to Australia in about 4.6 hours, with a price tag similar to what you would pay for business class these days. This speed would heat the body of the craft so that windows are not an option because the appropriate thickness would represent a considerable weight. It is therefore thanks to flat panel displays showing images that you would be able to enjoy the scenery.
When one talks about high-velocity flight it is difficult not to think of the French Concorde that operated between 1976 and 2003 and could travel at Mach 2.04 (limited by thermal constraints due to the material used) using the Scramjet technology; scramjet standing for “supersonic combustion ramjet”. This allowed a New York City to Paris flight in less than 3.5 hours instead of 8 hours with a conventional jet.
The principle of this technology is to compress air with shock waves under the body of the aircraft before injecting the fuel (the Concorde’s intake ramp system can be seen on the figure on the right).
Due to the high inefficiency of this technology at low speeds, afterburners are used from take-off until reaching the upper transonic regime.
Keeping in mind that the heating of the Concorde’s body due to friction could make it expand by as much as close to a foot, it becomes easy to understand one of the reasons why high altitudes (scarcer air and therefore lesser aerodynamic resistance) are chosen for such high flight velocities; the Concorde cruising altitude was around 56,000 ft and would be decreased when sun radiation levels were becoming too high. On a side note you can keep an eye out at Charles de Gaulle airport in Paris (France) for a Concorde displayed outside.
Oh and did I forget to mention that the turbomachinery parts on the SABRE engine are currently being designed in the AxSTREAM suite??